The present invention relates to a sintered beta-alumina article permeable to sodium and potassium ions and method for producing same. The article consists of a .beta. -alumina polycrystal sintered body including tantalum which can be used as a solid electrolyte or membrane for a sodium-sulfur storage battery, or the like.
Beta-alumina is a crystalline material of hexagonal structure having the chemical formula of Na.sub.2 O.11Al.sub.2 O.sub.3, and it possesses the characteristic that sodium ions can be selectively moved therethrough in the direction perpendicular to C-axis of the crystal. The special resistance of the movement of sodium ions in said direction is about 3.5.OMEGA. cm at the temperature of 300.degree. C. It is also known that a beta-alumina polycrystal sintered article, obtained by sintering beta-alumina single crystal powder, has the same ion-permeability as said single crystal material, and sodium ions move through said article in an electric field. Therefore, a beta-alumina polycrystal sintered article (hereinafter, the article wll be called a beta-alumina sintered article) is used as a sodium ion-permeable article such as a solid electrolyte in a sodium-sulfur storage battery in which molten sodium is employed as a negative electrode and molten sulfur is employed as a positive electrode.
A sodium-sulfur battery, based on electromotive reaction, operates as follows: During discharge, sodium ions move toward the positive electrode through the solid electrolyte, and sodium polysulfide is formed at the positive electrode, electrons being supplied thereto through an external circuit.
Therefore, an ion-permeable article employed as a solid electrolyte for such a storage battery, should have a large electric conductivity and a dense structure, and its quality should not change during repeated charging and discharging. However, a conventional beta-alumina sintered body consisting of Na.sub.2 O, Li.sub.2 O and Al.sub.2 O.sub.3, or of Na.sub.2 O, Li.sub.2 O, MgO and Al.sub.2 O.sub.3 has such drawbacks that when charging and discharging are repeatedly carried out, its ion-permeability is lowered after a relatively short period of time, that is, its function as a solid electrolyte cannot be maintained for a long period of time. The mechanism for moving sodium ions in a beta-alumina sintered body is not well understood, but considering that the sintered body consists of a polycrystal which is a combination of single crystals, it is assumed that sodium ions move in the direction perpendicular to C-axis of the respective single crystals. Since each single crystal in the sintered body forms a grain, sodium ions which have moved perpendicularly to the C-axis of one single crystal must pass through a grain in order to reach the adjacent grain. Usually, minute impurities which are pushed out of the crystals as a result of grain growth during sintering are distributed along the grain boundaries. When these impurities are ones which are easily attacked by sodium ions, such as oxides of iron, titanium and silicon, it is assumed that the grain boundaries are gradually damaged while the sodium ions pass therethrough, resulting in the destruction of the boundaries. Using a beta-alumina sintered article as a solid electrolyte for a sodium-sulfur storage battery, it was observed under an electron microscope that the surface of the sintered article was broken and destroyed by repetitive charging and discharging, and it was found that the destruction occurred at the grain boundaries, thus proving the above-mentioned assumption. Although it is desirable that the beta-alumina powder used as the base material for the sintered body be as pure as possible in order to prevent such destruction, it is almost impossible to exclude minute impurities.